This invention relates to a system for carrying out predictive encoding of an autocorrelated signal into a prediction error signal or predictive decoding of a prediction error signal derived from an autocorrelated signal into a reproduction of the last-mentioned autocorrelated signal. An example of the autocorrelated signals to which this invention is applicable is a television signal.
In a television signal, picture signals have a strong degree of correlation along both horizontal and vertical directions of each picture and along the time axis. It is therefore possible by resorting to predictive encoding and decoding to save the amount of information actually transmitted. According to predictive encoding applied to the picture signals, each picture element is represented by that instantaneous amplitude of a prediction error signal which is given by a difference between an unprocessed signal for the picture element and a prediction signal derived for the picture element from a signal for another picture element adjacent to the element under consideration. In intraframe predictive encoding, the prediction signal is derived from a signal for a picture element within the same frame. In interfield and interframe predictive encoding, a picture element corresponding in a next previous field and frame to the picture element in question is used, respectively, to derive the prediction signal. Use is also possible of a composite prediction signal obtained by a combination of the above-named modes of predictive encoding. The intraframe predictive encoding develops prediction signals excellently predictive of the picture elements of a picture of a less sharp contrast to provide a high efficiency. The interframe predictive encoding shows an excellent efficiency when applied to pictures for an object that little moves. The interfield and composite predictive encoding has a feature intermediate between the intraframe and interframe predictive encoding.
In a conventional predictive encoder for a television signal, use is generally made of only one of the various manners of predictive encoding. This is objectionable for a television broadcast signal in which the scene being televised may little or severely vary with respect to time and may be either simple or complicated. In "Conference Record," Volume II, pages 27-12 et seq., of International Conference on Communications held June 16-18, 1975, in San Francisco, Calif., the United States, a predictive encoder is proposed with reference to FIG. 4 (page 27-13). The proposed encoder is operable both in interframe and intraframe encoding modes. A mode controller checks which of the modes provides a higher efficiency for the time being and selects the more efficient mode. With this encoder, it is necessary besides the prediction error signal to transmit the information representative of the mode in which the transmission is in progress.
A television signal is an autocorrelated signal of a type that takes those values at equally spaced successive instants, such as instants of transmission of picture elements, which are substantially correlated to the values the autocorrelated signal takes each of at least two predetermined durations or periods after the respective instants. For a television signal, the two predetermined durations may be any two of an interval between two adjacent ones of the picture elements, a horizontal line period, a field period, a frame period, and a predetermined number of the field and/or frame periods. It is therefore possible to specify for an autocorrelated signal of the type described that a first of the predetermined durations is equal at least to the equal spacing between two adjacent ones of the instants and that a second of the predetermined durations is longer than the first predetermined duration. The prediction error signal as named herein has an amplitude variable from one of the instants to another in compliance with the autocorrelated signal.